1. Field of the Invention
The present invention generally relates to plasma processing of semiconductor wafers in the manufacture of integrated circuits (ICs) and, more particularly, to a plasma processing device using an optimized helical resonator to achieve efficient processing.
2. Description of the Prior Art
Plasma processing is an indispensable technology in the manufacture of very large scale integrated (VLSI) circuits. Plasma-assisted etching techniques have replaced chemical etching, and sputtering has replaced evaporation for the deposition of metals. Plasma enhanced chemical vapor deposition (PECVD) is an alternative to conventional and low pressure CVD techniques. In a reactive ion etcher (RIE), for example, silicon wafers are loaded on a radio frequency (RF) powered electrode and the chamber is grounded. In typical sputtering and PECVD devices, the wafers are loaded onto a revolving turntable in the chamber.
A number of schemes have been used to generate the plasma used for processing the wafers, including magnetrons and capacitive devices. More recently, helical resonators (i.e., inductors) have been used to generate the plasma. An example is disclosed, for example, in U.S. Pat. No. 4,368,092 to Steinberg et al. This device uses a downstream, high Q helical resonator to produce the plasma that is introduced to the reaction chamber. A disadvantage of the Steinberg et al. device is that it is radially asymmetric resulting in nonuniformities of the etching of the workpiece. U.S. Pat. No. 4,918,031 to Flamm et al. discloses another example of a high Q, helical resonator wherein, in one embodiment, the substrates are held within the resonator, thus avoiding the nonuniformities produced by the Steinberg et al. device. However, because of the high Q, the source efficiency is low, and wall sputtering of the source is increased, leading to damage to the chamber and particulate generation.
The ratio of the reactance (inductive or capacitive) of a circuit to the series resistance of the circuit is called the Q (for quality factor) of the circuit. Helical resonators for producing plasmas are typically high Q devices with an unloaded Q (i.e., plasma attenuated) of about 1000. These devices are capable of producing large voltages and can produce plasmas at low pressures. However, these same properties can cause them to operate in a capacitively coupled mode instead of an inductive mode. Capacitive coupling is much less efficient than inductive coupling for generating high plasma densities at low pressures (e.g., 1-20 mTorr).